1. Field of the Invention
The present invention generally relates to a method and apparatus for manufacturing solder balls and, more particularly, to a method and apparatus for manufacturing solder balls for BGA (Ball Grid Array) used recently for electronic packaging.
2. Description of the Prior Art
As is well known to those skilled in the art, conventional methods for manufacturing tiny metal balls may be classified into three types: a centrifugal spraying type method, a gas-related type method and a method wherein a metal piece having a uniform weight is molten and forms many balls in liquid and the balls are solidified again. However, with the centrifugal spraying type method and the gas-related type method, ball size is not uniform due to their manufacturing characteristics. Also, a very large apparatus is required for solidifying the balls in gas because the balls are greatly accelerated in the manufacturing process. On the other hand, according to the method wherein a metal piece having a uniform weight is molten and forms many balls in liquid and the balls are solidified again, many processes are required prior to the solidification because metal pieces having uniform weights must be made, thus increasing manufacturing cost. In brief, the conventional methods have low gaining or yield rates and are not economical.
There is another method wherein molten metal flows downward through a small orifice. This method uses a natural phenomenon in which liquid in a fine flow is in an unstable state due to boundary surface tension and the liquid is easily cut and forms many droplets by slight waves on a flowing surface. Lord Rayleigh analyzed this phenomenon and explained that the wavelength of the most unstable wave is about 1.9 times as large as the diameter of the orifice in an article "On the Instability of Jets", London Mathematical Society, 1878. Accordingly, the diameters of the generated droplets have a normal distribution centering at the diameter about 1.9 times as large as the diameter of the orifice. In such a case, when a vibration having a uniform frequency is applied to the liquid, waves having a uniform periodic time are generated on the surface of the liquid passing through the orifice and the waves are cut into fractions having a uniform size, thereby forming many metal balls.
Such a technique using the natural phenomenon is disclosed in "Inkjet Printer of IBM, J, Resent Development" published in 1974, the method of vibrating a disc in molten metal of U.S. Pat. No. 5,266,098 and the method of applying vibrations from the exterior of liquid to the liquid using a speaker of "Power and Powder metallurgy", Vol. 38, No. 6 published in Japan in 1991. However, this method requires a large apparatus because small droplets are easily solidified in the atmosphere, but large droplets need a long falling distance so as to be solidified, may not regulate the size of balls and reduces a gaining rate.
In order to overcome this defect, the method of solidifying droplets using cooling liquid is proposed in "The International Journal of Power Metallurgy", Vol. 32 published in 1996. However, according to this method, since molten metal is boiled at the surface of a droplet upon a droplet falling into cooling liquid, the shape of the ball is determined in accordance with the boundary surface tension and the size of the molten metal, thus deteriorating the surface quality and the degree of sphericity of ball.